Neural precursor cell-Expressed Developmentally Down regulated gene 9 (NEDD9;HEF1, CAS-L) is a scaffolding protein regulating mitosis, survival, and migration. Within the last 3 years, NEDD9 has been implicated in the progression and metastasis of several cancers. Recently, a viable and fertile Nedd9 knockout strain has been developed that enables evaluation of the in vivo role of NEDD9 in cancer initiation and progression. Nedd9-/- mice were crossed to MMTV-neu (HER2;ErbB2) transgenic mice, and compared with MMTV-neu;Nedd9+/+ counterparts. Strikingly, by 18 months, 88% of MMTV-neu;Nedd9+/+ mice developed tumors, in contrast to only 18% of MMTV-neu;Nedd9-/- mice. This indicates a novel role for NEDD9 in the initiation of mammary tumor genesis, in contrast to progression. The project goal is to understand why the Nedd9-/- genotype reduces tumor incidence. MMTV-neu tumors commonly develop missense mutations in p53 to enhance tumor formation and progression after a significant period of latency (7-12 months), paralleling the observed functional loss of p53 in human HER2-overexpressing tumors. Aurora A kinase (AURKA) has recently emerged as a negative regulator of p53 activity and stability. Work from our laboratory has recently shown NEDD9 synergizes with a second protein, Ajuba, to positively and directly regulate AURKA. We have shown that siRNA-mediated knockdown of NEDD9 not only inhibits AURKA activity, but lowers AURKA protein levels. These results imply that in MMTV-neu;Nedd9-/- mammary epithelial cells, there would be low levels of AURKA protein, with commensurate failure to inhibit p53. In turn, early MMTV- neu;Nedd9-/- mammary epithelium would retain p53-dependent tumor suppression longer than the MMTV-neu;Nedd9+/+ epithelial cells, causing resistance to tumor formation. This proposal aims to investigate the NEDD9-AURKA interaction in MMTV-neu-driven mammary tumorigenesis. We will use techniques to investigate the status and functional implications of AURKA-p53 feedback in reference to the links already established between NEDD9, AURKA, and Ajuba. In addition, we will determine whether Nedd9 status affects therapeutic sensitivity of MMTV-neu tumors to AURKA inhibitors.